Wind power plant

ABSTRACT

The magnetic circuit of a generator in a wind power plant is arranged to directly supply a high supply voltage of 2-50 kV, preferably higher than 10 kV. The generator is provided with solid insulation and its winding includes a cable ( 6 ) comprising one or more current-carrying conductors ( 31 ) with a number of strands ( 36 ) surrounded by at least one outer and one inner semiconducting layer ( 34, 32 ) and intermediate insulating layers ( 33 ). The outer semiconducting layer ( 34 ) is at earth potential. The stator winding may be produced with full or fractional slot winding, the phases of the winding being Y-connected. The Y-point may be insulated and protected from over-voltage by means of surge arrestors, or else the Y-point may be earthed via a suppression filter. The invention also relates to a wind power plant, a generator included in the plant and a variable speed system for such a plant.

TECHNICAL FIELD

This invention relates to a wind power plant which is intended forconnection to distribution or transmission networks, hereinafrer calledpower networks. The invention also relates to an electric generator forhigh voltage in a wind power station intended for the above-mentionedpurpose. The invention further relates to a variable speed systemcontaining the above-mentioned generator.

BACKGROUND ART

A wind power plant can be a single grid-connected unit but usuallyconsists of a number of wind turbines forming a wind power farm. Eachwind turbine is equipped with an electric generator located in a hub.The generator can be synchronous or of the induction type. Inductiongenerators are more common today because they are cheaper and morerobust. The synchronous generator can produce reactive power which is anadvantage over the induction machine. The size of the wind turbine istoday typically 100-3000 kW with many commercial turbines around 500 kw.The trend is for higher power and voltage of the generator. The voltagelevels of today are from 400 V up to a few kV. In most wind farms, it isnecessary to equip each wind turbine with a transformer that steps upthe voltage to a local distribution voltage that may be typically 10-30kV. Thus this transformer and the generator constitute integrated partsof a plant. Individual units are interconnected in tree branch or ringnetworks with high-voltage cables. The distribution network may beconnected to a transmission network by a single or a couple of powertransformers. The transformers entail an extra cost and also have thedrawback that the total efficiency of the system is reduced. They arealso a fire hazard since they contain transformer oil which can leak outin the event of failure or vandalism.

If, therefore, it were possible to manufacture electric generators forconsiderably higher voltages, at least the distribution transformercould be eliminated. It is possible with today's generator technology tomake a 10 kV generator and thus eliminate the distribution transformer,but the cost would be far higher than a more typical 660 V machine.Furthermore today's stator winding insulation technology is sensitive totemperature variations, humidity and salt that a wind turbine generatormay be exposed to. This makes it unrealistic with today's technology todispose of the distribution transformers.

A high-voltage generator has a magnetic circuit that comprise alaminated core, e.g. of sheet steel with a welded construction. Toprovide ventilation and cooling the core is often divided into stackswith radial and/or axial ventilation ducts. The winding of the magneticcircuit is disposed in slots in the core, the slots generally having across section in the shape of a rectangle or trapezium.

In multi-phase high-voltage electric generators the windings are made aseither single or double layer windings. With single layer windings thereis only one coil side per slot, whereas with double layer windings thereare two coil sides per slot. By “coil side” is meant one or moreconductors combined vertically or horizontally and provided with acommon coil insulation, i.e. an insulation designed to withstand therated voltage of the generator to earth.

Double-layer windings are generally made as diamond windings whereassingle layer windings in the present context can be made as diamond orflat windings only one (possibly two) coil width exists in diamondwindings whereas flat windings are made as concentric windings, i.e.with a widely varying coil width. By “coil width” is meant the distancein arc dimension between two coil sides pertaining to the same coil.

Normally all large machines are made with doublelayer windings and coilsof the same size. Each coil is placed with one side in one layer and theother side in the other layer. This means that all coils cross eachother in the coil end. If there are more than two layers these crossingscomplicate the winding work and the coil end is less satisfactory.

It is considered that coils for rotating generators can be manufacturedwith good results within a voltage range of 3-20 kV.

In theory, it is known how to obtain larger voltage levels. Suchgenerators are described, for instance, in US-A-4429244, US-A-4164672and US-A-3743867. However, the machine designs according to the abovepublications do not permit optimal utilization of the electromagneticmaterial in the stator.

There are also wind turbines that operate at variable turbine speed.This operation mode is advantageous because the aerodynamic efficiencycan be maximized. Variable speed systems employ two generators withdifferent numbers of poles or generators with windings that can beconnected for two-speed operation. Variable speed can also be obtainedby means of a frequency converter. A variable speed system is simplifiedwhen a synchronous generator is used because a simple diode rectifiercan be used between generator and DC-link. The two most common invertertypes are line commutated and force-commutated. These two types ofinverters produce different types of harmonics and hence requiredifferent line filters. The line-commutated inverter is equipped withthyristors which produces harmonic current that are turned into voltageharmonics on the grid. To eliminate these harmonics a large grid filtermust be used. Another drawback is that the line-commutated inverterconsumes reactive power. A force-commutated inverter can create its ownthree-phase voltage system and if the inverter is connected to the gridit can freely choose which power factor to use and in which directionthe power should be directed. By the use of Pulse Width Modulation, PWM,the low frequency harmonics are eliminated and the first harmonics havea frequency around the switching frequency of the inverter. The mostinteresting valve for a PWM inverter is the insulated Gate BipolarTransistor, IGBT. With the latest IGBT-valves, a switching frequency offrom 5 to 10 kHz would be used. Today's IGBT valves are limited involtage and power so that a single six-pulse inverter can handle about 1MVA at 1-2 kV.

DESCRIPTION OF THE INVENTION

The object of the invention is thus to provide an electric generatorwhich can be used in a wind power plant for such high voltage that thedistribution transformer can be omitted, i.e. a plant in which theelectric generators are intended for considerably higher voltages thanconventional machines of corresponding type, in order to be able toexecute direct connection to power networks at all types of highvoltages, in particular exceeding the 20 kV considered as an upper limittoday. Another object of the invention is to provide an electricgenerator that is not sensitive to salt, humidity or temperaturevariations, as are present known high-voltage windings. A third objectof the invention is to provide a variable speed alternative for theresulting high voltage if the distribution transformer is eliminated.

By use of solid insulation in combination with the other featuresdefined, the network can be supplied without the use of an intermediatestep-up transformer even at network voltages considerably in excess of20 kV. Furthermore, this insulation is completely insensitive to salt,humidity and temperature variations. The elimination of the transformerentails great savings and also results in several other simplificationsand savings.

Wind power plants are often arranged in farmland and close to populatedareas. In a conventional wind power plant the transformer must beprotected from causing hazard by explosion risk or leaking oil. Aconcrete transformer station may have to be built at the foundation ofeach wind turbine unit. In future offshore locations it would bedifficult and costly to repair and maintain the transformer. Thus if thetransformer is eliminated, the transformer housing is eliminated and itis also possible to use thinner cables to the generator. Furthermore thereactive power consumption and the electrical losses of the transformerare eliminated. The removal of the transformer also eliminates a set ofbreaker units previously necessary between the transformer and thegenerator.

The plant according to the invention also enables several connectionswith different voltage levels to be arranged, i.e. the invention can beused for all auxiliary power in the power station. Another way to supplyauxiliary power to each wind turbine is to have a cheap low-voltagenetwork in parallel with the distribution network.

According to another aspect of the present invention there is providedan electric generator as claimed in the ensuing claim 25.

In a particularly preferred embodiment of the plant and generatorrespectively, the solid insulation system comprises at least two spacedapart layers, e.g. semiconducting layers, each layer constitutingessentially an equipotential surface, and an intermediate solidinsulation therebetween, at least one of the layers having substantiallythe same coefficient of thermal expansion as the solid insulation.

This embodiment constitutes an expedient embodiment of the solidinsulation that in an optimal manner enables the windings to be directlyconnected to the high-voltage network and where harmonization of thecoefficients of thermal expansion eliminates the risk of defects, cracksor the like upon thermal movement in the winding.

It should be evident that the windings and the insulating layers areflexible so that they can be bent. It should also be pointed out thatthe plant according to the invention can be constructed using eitherhorizontal or vertical generators.

A major and essential difference between known technology and theembodiment according to the invention is that an electric generator witha magnetic circuit is arranged to be directly connected via onlybreakers and isolators, to a high supply voltage, typically in thevicinity of between 2 and 50 kv, preferably higher than 10 kV. Themagnetic circuit comprises a laminated core having at least one windingconsisting of a threaded cable with one or more permanently insulatedconductors having a semiconducting layer both at the conductor andoutside the insulation, the outer semiconducting layer being connectedto earth potential.

To solve the problems arising with direct connection of electricmachines to all types of high-voltage power networks, the generator inthe plant according to the invention has a number of features asmentioned above, which differ distinctly from known technology.Additional features and further embodiments are defined in the dependentclaims and are discussed in the following.

Such features mentioned above and other essential characteristics of thegenerator and thus of the wind-power plant according to the inventioninclude the following:

The winding of the magnetic circuit is produced from a cable having oneor more permanently insulated conductors with a semiconducting layer atboth conductor and sheath. Some typical conductors of this type are XLPEcable or a cable with EP rubber insulation which, however, for thepresent purpose are further developed both as regards the strands in theconductor and the nature of the outer sheath.

Cables with circular cross section are preferred, but cables with someother cross section may be used in order, for instance, to obtain betterpacking density.

Such a cable allows the laminated core to be designed according to theinvention in a new and optimal way as regards slots and teeth.

The winding is preferably manufactured with insulation in steps for bestutilization of the laminated core.

The winding is preferably manufactured as a multilayered, concentriccable winding, thus enabling the number of coil-end intersections to bereduced.

The slot design is suited to the cross section of the winding cable sothat the slots are in the form of a number of cylindrical openingsrunning axially and/or radially outside each other and having an openwaist running between the layers of the stator winding.

The design of the slots is adjusted to the relevant cable cross sectionand to the stepped insulation of the winding. The stepped insulationallows the magnetic core to have substantially constant tooth width,irrespective of the radial extension.

The above-mentioned further development as regards the strands entailsthe winding conductors consisting of a number of impacted strata/layers,i.e. insulated strands that from the point of view of an electricmachine, are not necessarily correctly transposed, uninsulated and/orinsulated from each other.

The above-mentioned further development as regards the outer sheathentails that at suitable points along the length of the conductor, theouter sheath is cut off, each cut partial length being connecteddirectly to earth potential.

The use of a cable of the type described above allows the entire lengthof the outer sheath of the winding, as well as other parts of the plant,to be kept at earth potential. An important advantage is that theelectric field is close to zero within the coil-end region outside theouter semiconducting layer. With earth potential on the outer sheath theelectric field need not be controlled. This means that no fieldconcentrations will occur either in the core, in the coil-end regions orin the transition between them.

The mixture of insulated and/or uninsulated impacted strands, ortransposed strands, results in low stray losses. The cable for highvoltage used in the magnetic circuit winding is constructed of an innercore/conductor with a plurality of strands, at least two semiconductinglayers, the innermost being surrounded by an insulating layer, which isin turn surrounded by an outer semiconducting layer having an outerdiameter in the order of 10-40 mm and a conductor area in the order of10-200 mm².

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described in more detail, byway of example only, with particular reference to the accompanyingdrawings, in which

FIG. 1 is a schematic axial end view of a sector of the stator of anelectric generator of a wind power plant according to the invention,

FIG. 2 is an end view, partially stripped, of a cable used in thewinding of the stator according to FIG. 1,

FIG. 3 is a simplified view, partially in section, of a wind-powergenerator arrangement according to the invention, and

FIG. 4 is a circuit diagram for the wind-power plant according to theinvention,

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 shows part of a stator 1 and rotor 2 of a generator 100 (see FIG.3) of a wind power plant according to the invention. The stator 1comprises, in conventional manner, a laminated core. FIG. 1 shows asector of the generator corresponding to one pole pitch. From a yokepart 3 of the core situated radially outermost, a number of teeth 4extend radially in towards the rotor 2 and are separated by slots 5 inwhich the stator winding is arranged. Cables 6 forming this statorwinding, are highvoltage cables which may be of substantially the sametype as those used for power distribution, i.e. XLPE (crosslinkedpolyethylene) cables. One difference is that the outer,mechanically-protective PVC-layer, and the metal screen normallysurrounding such power distribution cables are eliminated so that thecable for the present application comprises only the conductor, aninsulating layer and at least one semiconducting layer on each side ofthe insulating layer. The cables 6 are illustrated schematically in FIG.1, only the conducting central part of each cable part or coil sidebeing shown. As can be seen, each slot 5 has a varying cross sectionwith alternating wide parts 7 and narrow parts 8. The wide parts 7 aresubstantially circular and surround the cabling, the waist parts betweenthese forming narrow parts 8. The waist parts serve to radially fix theposition of each cable. The cross section of the slot 5 also narrowsradially inwards. This is because the voltage on the cable parts islower the closer to the radially inner part of the stator 1 they aresituated. Thinner cabling can therefore be used there, whereas widercabling is necessary radially further out. In the example illustratedcables of three different dimensions are used, arranged in threecorrespondingly dimensioned sections 51, 52, 53 of slots 5. An auxiliarypower winding 9 is arranged furthest out in the slot 5.

FIG. 2 shows a step-wise stripped and view of a high-voltage cable foruse in the present invention. The high-voltage cable 6 comprises one orore conductors 31, each of which comprises a number of strands 36, e.g.of copper, which together form a central conducting means of generallycircular cross section, which may be insulated. At least one of thestrands 36A is uninsulated. These conductors 31 are arranged in themiddle of the high-voltage cable 6 and in the shown embodiment each issurrounded by a part insulation 35. However, it is feasible for the partinsulation 35 to be omitted on one of the conductors 31. In the presentembodiment of the invention the conductors 31 are together surrounded bya first semiconducting layer 32. Around this first semiconducting layer32 is a solid insulating layer 33, e.g., XLPE insulation, which is inturn surrounded by a second semiconducting layer 34. Thus the concept“high-voltage cable” in this application need not include any metallicscreen or outer PVC-layer of the type that normally surrounds such acable for power distribution.

A wind-power plant with a magnetic circuit of the type described aboveis shown in FIG. 3 where the generator 100 is driven by a wind turbine102 via a shaft 101 and a gearbox 114. The stator 1 of the generator 100carries stator windings 10 which are built up of the cable 6 describedabove. The cable 6 is unscreened and changes to a screened cable 11 atcable splicing 9.

FIG. 4 illustrates a wind power plant according to the presentinvention. In conventional manner, the generator 100 has an excitationwinding 112 and one (or more) auxiliary power winding(s) 113. In theillustrated embodiment of the plant according to the invention thegenerator 100 is Y-connected and the neutral earthed via an impedance103. It can also be seen from FIG. 4 that the generator 100 iselectrically connected via the cable splicing 9 to the screened cable 11(see also FIG. 3). In some cases it would be possible to omit the cablesplicing and let the generator cable extend down the tower of the windturbine. The cable 11 is provided with current transformers 104 inconventional manner, and terminates at 105. After this point 105 theelectric plant in the embodiment shown continues with busbars 106 havingbranches with voltage transformers 107 and surge arresters 108. However,the main electric supply takes place via the busbars 106 directly to thedistribution or transmission network 110 via isolator 109 andcircuit-breaker 111.

Although the generator and the plant in which this generator is includedhave been described and illustrated in connection with an embodiment byway of example, it should be obvious to one skilled in that art thatseveral modifications are possible without departing from the inventiveconcept. The gearing may be omitted if using a low-speed generator. Thegenerator may be earthed directly without any impedance. The auxiliarywindings can be omitted, as also can other components shown. Althoughthe invention has been exemplified with a three-phase plant, the numberof phases may be more or less. The generator can be connected to thegrid via a frequency convertor containing a rectifier, a DC-link and aninverter. Unlike conventional variable-speed systems, the valves of therectifier and inverter would probably have to be series-connectedbecause of the high voltage.

Although it is preferred that the electrical insulation system for thewinding should be extruded in position, it is possible to build up anelectrical insulation system from tightly wound, overlapping layers offilm or sheet-like material. Both the semiconducting layers and theelectrically insulating layer can be formed in this manner. Aninsulation system can be made of an allsynthetic film with inner andouter semiconducting layers or portions made of polymeric thin film of,for example, PP, PET, LDPE or HDPE with embedded conducting particles,such as carbon black or metallic particles and with an insulating layeror portion between the semiconducting layers or portions.

For the lapped concept a sufficiently thin film will have butt gapssmaller than the so-called Paschen minima, thus rendering liquidimpregnation unnecessary. A dry, wound multilayer thin film insulationhas also good thermal properties.

Another example of an electrical insulation system is similar to aconventional cellulose based cable, where a thin cellulose based orsynthetic paper or non-woven material is lap wound around a conductor.In this case the semiconducting layers, on either side of an insulatinglayer, can be made of cellulose paper or non-woven material made fromfibres of insulating material and with conducting particles embedded.The insulating layer can be made from the same base material or anothermaterial can be used.

Another example of an insulation system is obtained by combining filmand fibrous insulating material, either as a laminate or as co-lapped.An example of this insulation system is the commercially availableso-called paper polypropylene laminate, PPLP, but several othercombinations of film and fibrous parts are possible. In these systemsvarious impregnations such as mineral oil can be used.

In this specification “semiconducting material” means a substance whichhas a considerably lower conductivity than an electric conductor butwhich does not have such a low conductivity that it is an electricinsulator. Suitably, but not essentially, the semiconducting materialwill have a resistivity of 1-10⁵ ohm-cm, preferably 10-500 ohm-cm andmost preferably from 10 to 100 ohm-cm, typically 20 ohm-cm.

1. A wind power plant comprising at least one high voltage rotarygenerator coupled to a turbine via shaft means and having a stator withat least one winding and a rotor, wherein the at least one statorwinding comprises: a cable including a current carrying conductor, aninner layer having semiconducting properties surrounding the currentcarrying conductor, a solid insulation layer surrounding the inner layerand an outer layer having semiconducting properties surrounding thesolid insulation layer, wherein the current-carrying conductor comprisesa plurality of electrically insulated strands and at least oneuninsulated strand in contact with the inner layer.
 2. The plant asclaimed in claim 1, wherein the inner and outer layers each provideessentially an equipotential surface, and the insulating layer hassubstantially the same coefficient of thermal expansion as thesemiconducting layers.
 3. The plant as claimed in claim 1, wherein thelayer is at substantially the same potential as the said conductor. 4.The plant as claimed in claim 1, wherein the outer semiconducting layeris arranged to form essentially an equipotential surface surrounding theconductor.
 5. The plant as claimed in claim 1, wherein said outersemiconducting layer is connected to a predefined potential.
 6. Theplant as claimed in claim 5, the predefined potential is earthpotential.
 7. The plant as claimed in claim 1, wherein the rotor isequipped with a short-circuited winding, resulting in a generator of theinduction type.
 8. The plant as claimed in claim 1, wherein the rotor isequipped with a field winding in which DC-current flows, resulting in agenerator of the synchronous type.
 9. The plant as claimed in claim 1,wherein the conductor has a conductor area of between 10 and 200 mm² andthe cable has an outer cable diameter of between 10 and 40 mm.
 10. Theplant as claimed in claim 1, wherein the said generator is designed forhigh voltage and is arranged to supply the out-going electric networkdirectly without any intermediate connection of a transformer.
 11. Theplant as claimed in claim 10, wherein said generator is earthed via animpedance.
 12. The plant as claimed in claim 10, wherein said generatoris directly earthed.
 13. The plant as claimed claim 10, wherein thegenerator is arranged to generate power to various voltage levels. 14.The plant as claimed claim 13, wherein one of said voltage levels isarranged to generate auxiliary power and that the auxiliary power isarranged to be generated from a separate winding in the generator. 15.The plant as claimed in claim 1 wherein it comprises several generators,each of which lacks an individual step-up transformer, but which, via asystem transformer common to the generators, is connected to thetransmission or distribution network.
 16. The plant as claimed in claim1 wherein the winding of the or each generator is arranged forself-regulating field control and lacks auxiliary means for control ofthe field.
 17. The plant as claimed in claim 1 wherein the windings ofthe or each generator can be connected for multiple-speed operationusing different numbers of poles.
 18. The plant as claimed in claim 1wherein at least one wind turbine is equipped with two or moregenerators having different numbers of poles so that multiple-speedoperation is possible.
 19. The plant as claimed in claim 1 wherein theor each generator is connected to a frequency convertor comprising arectifier, a DC-link and an inverter.
 20. The plant as claimed in claim19, wherein series connected valves are used in the inverter and therectifier.
 21. The plant as claimed in claim 20, wherein the inverter isnet commutated with current-stiff DC-link.
 22. The plant as claimed inclaim 20, wherein the inverter is self commutated and comprisesseries-connected IGBTs.
 23. The wind power plant according to claim 1,including coupling means for connecting the plant to a transmission ordistribution network having a voltage of between 2 and 50 kV.
 24. Anelectric generator for high voltage included in a wind power plant inwhich the generator is coupled to a turbine via shaft means, saidgenerator comprising a stator with at least one stator wherein the atleast one stator winding comprises a cable including a current carryingconductor, an inner semiconducting layer surrounding the conductor, asolid insulation layer surrounding the inner layer and an outersemiconducting layer surrounding the solid insulation, wherein thecurrent-carrying conductor comprises a plurality of electricallyinsulated strands and at least one uninsulated strand in contact withthe inner layer.
 25. The electric generator of claim 24 includingcoupling means for coupling the generator directly to a transmission ordistribution network having a voltage of between 2 to 50 kV.